Most commercial alumina is produced according to the "Bayer Process", a well known process for the production of alumina from bauxite. In the Bayer Process, bauxite is mixed with hot concentrated NaOH which reacts with and puts into solution some of the alumina, silica and other components of the bauxite. Most of the silica and other materials are reprecipitated and eliminated as a solid waste referred to as "red mud". The dissolved alumina is then separated in solution from remaining solids and crystalized as alumina trihydrate, .alpha.-Al.sub.2 O.sub.3.3H.sub.2 O ("gibbsite"). Because it is formed in a sodium hydroxide environment, the gibbsite contains a significant amount (usually 0.3 to 0.4%) soda, Na.sub.2 O. (All percentages herein are by weight unless otherwise noted.) In addition, the economics of the Bayer Process are such that significant amounts of other impurities such as silica are tolerated in the gibbsite product. A typical analysis of gibbsite from the Bayer Process is shown in Table 1. In accordance with the usual practice, impurities are expressed as the stable oxide form.
TABLE 1 ______________________________________ Impurities Concentration, % ______________________________________ Na.sub.2 O 0.3 SiO.sub.2 0.01-0.04 CaO 0.025 Ga.sub.2 O.sub.3 0.01 Fe.sub.2 O.sub.3 0.01 ______________________________________
A number of other minor oxides are also present, in quantities of less than a few hundred ppm each. When the Bayer Process alumina trihydrate is calcined to produce anhydrous alumina, Al.sub.2 O.sub.3, the impurities are concentrated by a factor of about 1.5.
While most of the commercial hydrated alumina is produced by the Bayer Process as described, it is possible to produce hydrated alumina by other methods. To the extent that such other methods result in the inclusion of unacceptably high levels of silica in the hydrated alumina, the purification process of the present invention will be applicable to purification of those materials. For brevity herein, however, the process of this invention will be described in terms of the purification of Bayer Process-produced gibbsite, although it is to be understood that it is applicable to all hydrated aluminas containing silica as an impurity.
For most alumina uses, such as electrolytic production of aluminum metal or formation of ordinary ceramic products and refractories, the gibbsite is usable even with these high levels of impurities present. For a number of applications, however, these impurity levels (particularly the high soda and silica levels) are unacceptable. These applications include products intended for use as synthetic sapphire and as translucent bodies. Depending upon the particular application or product, maximum alumina impurity levels for materials such as soda, silica or iron oxide may be as low as 0.002% (20 ppm).
Previously, most aluminas of low soda content were derived from the gibbsite made by the Bayer Process. Reduction in soda levels was accomplished by one of several methods that could attain, at best, minimum Na.sub.2 O levels of 0.02-0.05% (200-500 ppm). Recently, however, a process has been developed by researchers of ARCO Metals Company in which, by a solid-to-solid reaction conducted in aqueous hydrochloric acid, the hydrated alumina is converted to aluminum chloride hexahydrate (ACH) and many the impurities, particularly the soda, are eliminated by conversion to soluble chlorides in the aqueous acid. This process (which will be referred to herein as the "ACH Process") has been described and claimed in a co-pending patent application assigned to the assignee of the present invention. Through the use of this process low soda content, high purity alumina can easily be made which is suitable for many applications.
While the ACH Process has been extremely effective for removal of those impurities in the hydrated alumina which can be converted to soluble chlorides, an important and common impurity, silica, does not readily react with the aqueous hydrochloric acid and therefore remains as a solid impurity dispersed in the solid ACH product. Since the initial silica content of the hydrated alumina is relatively low (particularly by comparison to the soda content) this silica impurity content level in the ACH product is unobjectionable for many applications. For numerous important applications, however, the resultant silica impurity content level is unacceptable. These applications are those which employ aluminas known in the industry as "super purity aluminas" ("SPA") which require a total impurity content of less than 0.01% (100 ppm). Typical SPA applications include synthetic sapphire, alumina optical glass and translucent tubing of the type used in sodium vapor lamps. In each of these applications it is important that the silica impurity content level in particular be minimized. For instance, in synthetic sapphire a typical specification requires not more than 0.003% (30 ppm) silica while in a typical specification for translucent tubing the maximum silica content is 0.005% (50 pm). Since the typical Bayer process hydrated alumina normally contains two to fifteen times more silica than those maxima, and since the silica impurity passes through the aforementioned ACH Process with no more than minor reduction, the resulting silica impurity level in the product ACH results after conversion in an ACH-derived alumina which is not suitable for SPA applications.
There is also a class of aluminas known in the industry as "high purity aluminas" ("HPA") which have less stringent impurity requirements than SPA, but which still represent significantly higher purity levels than ordinary aluminas. The invention herein is useful in producing HPA, but is most advantageously used for SPA production.
It would therefore be valuable to have a process which would serve not only to reduce the acid-soluble impurities to minimal levels but would also eliminate many of the acid-insoluble impurities (especially silica) in the alumina, such that the ultimate product alumina derived from the ACH would be acceptable in all respects for HPA and SPA applications.